The present invention relates to a photovoltaic device having a high efficiency.
Since it was found in 1976 by W. E. Spear et al that the conductivity of amorphous silicon obtained by plasma decomposition of silane (SiH.sub.4) could be greatly altered by doping with phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) and an amorphous silicon solar cell was fabricated on an experimental basis in 1976 by D. E. Carlson et al, the amorphous silicon thin film solar cell has attracted attention and a lively study of improvement in its efficiency has been made.
A PIN junction amorphous silicon solar cell has the disadvantages that the P or N type amorphous silicon has a short carrier lifetime and, therefore, fails to provide effective carriers, and also the P-layer suffers from large absorption loss of light because it has a higher light absorption coefficient and more non-radiative recombination centers than the I-layer.
In order to eliminate these disadvantages, an inverted PIN junction photovoltaic device has been proposed. This photovoltaic device is constructed so that light impinges on the N-type amorphous silicon side. Since the N-type amorphous silicon has a smaller light absorption coefficient than the P-type, it is considered to be somewhat advantageous.
In order to greatly improve the disadvantages as mentioned above of the PIN junction amorphous silicon solar cell, a heterojunction solar cell using amorphous silicon carbide in the P-layer located on the light impinging side was proposed by Y. Tawada, H. Okamoto and Y. Hamakawa in App. Phys. Lett., Vol. 39, pp 237-239 (1981). In this device, the light absorption loss due to the P-layer is greatly decreased by using amorphous silicon carbide, which has a large optical band gap, as a P-layer with valency controllability.
It is an object of the present invention to provide a PIN junction photovoltaic device.
A further object of the invention is to provide a photovoltaic device capable of more efficiently converting incident light energy into photo-current.
These and other objects of the present invention will become apparent from the description hereinafter.